1. Field of the Invention
The present invention relates to a dual-band/triple-band mobile radio terminal, a surface acoustic wave (hereinafter referred to as xe2x80x9cSAWxe2x80x9d) filter suitable for such a mobile radio terminal, or an antenna duplexer constituted by such an SAW filter and a high-frequency switch.
2. Description of the Related Art
In a mobile radio terminal, a single antenna is shared between transmission and reception through an antenna duplexer so as to transmit and receive transmission/reception signals adapted to a mobile communication system. A related-art antenna duplexer has a receiving dielectric resonator using a receiving frequency band as its passing frequency band, and a transmitting dielectric resonator using a transmitting frequency band as its passing frequency band. In addition, in the antenna duplexer, a matching circuit and a phase shift circuit are combined in order to prevent and reduce interference of the receiving and transmitting dielectric resonators with each other.
Mobile radio terminals small in size and light in weight have been under development for these years. Further, service which can use mobile radio terminals (dual-band or triple-band) adapted to a plurality of systems has been also in the planning stage. In such a state, in an antenna duplexer using dielectric resonators, its circuit wiring has become three-dimensional because the dielectric resonators are large in volume and heavy in weight. Thus, there has been a limit in making antenna duplexers and mobile radio terminals small in size and light in weight.
In order to cope with the requirement of making mobile radio terminals small in size and light in weight, an SAW antenna duplexer using SAW filters instead of dielectric resonators so as to miniaturize the antenna duplexer was developed, as disclosed in xe2x80x9cIEEE Proceeding of Ultrasonics Symposiumxe2x80x9d, p.19-24 (1996). FIG. 2 shows a circuit block diagram of such an antenna duplexer for a GSM system mobile radio terminal.
FIG. 1A shows a frequency allocation of a GSM system. A receiving frequency band fR ranges from 935 to 960 MHz, and a transmitting frequency band fT ranges from 890 to 915 MHz. An antenna duplexer 1 is connected to an antenna 2 through an antenna terminal 3 shared by a receiving system and a transmitting system. A reception signal supplied from the antenna 2 is supplied to the receiving system through the terminal 3 and a parallel connection point 4 which connects the receiving and transmitting systems. The reception signal is supplied to a receiving-system terminal 9 while desired receiving frequency characteristics are ensured. The receiving system includes an SAW filter 7 using the receiving frequency band fR as its passing frequency band, a matching circuit 6-1 for impedance-matching of the SAW filter 7, and a phase shift circuit 5-1 for suppressing leakage of a transmission signal into the receiving system. On the other hand, the transmission signal is supplied to the transmitting system through a transmitting-system terminal 10, and then supplied to the antenna 2 through the parallel connection point 4 and the terminal 3 while desired transmitting frequency characteristics are ensured. The transmitting system includes an SAW filter 8 using the transmitting frequency band fT as its passing frequency band, a matching circuit 6-2 for impedance-matching of the SAW filter 8, and a phase shift circuit 5-2 for suppressing leakage of the reception signal into the transmitting system.
In the related art, such an antenna duplexer used a dielectric single-layer substrate in which a signal pattern was formed on its upper surface and a ground pattern was formed on its lower surface, while elements constituting the SAW filters, phase shift circuits and matching circuits were mounted on its upper surface.
According to the present invention, a low-noise modulation system such as an off-set PLL modulation system is adopted and a transmitting system of an antenna duplexer is formed by a switch so that a mobile radio terminal suitable as a dual-band/triple-band one is realized. Particularly, in a triple-band mobile radio terminal, a transmitting system is shared by two mobile communication systems having transmitting frequency bands close to each other. Further, in mounting an SAW filter, a multi-layer substrate is adopted, and matching circuits and so on are formed by lumped-constant circuit elements, and the SAW filter is received in a space provided in the multi-layer substrate.